Quad Flat No Lead Package And Method Of Making

ABSTRACT

A quad flat no lead (“QFN”) package that includes a die having an active side positioned substantially in a first plane and a backside positioned substantially in a second plane parallel to the first plane; a plurality of separate conductive pads each having a first side positioned substantially in the first plane and a second side positioned substantially in the second plane; and mold compound positioned between the first and second planes in voids between the conductive pads and the dies. Also a method of producing a plurality of QFN packages includes forming a strip of plastic material having embedded therein a plurality of dies and a plurality of conductive pads that are wire bonded to the dies and singulating the strip into a plurality of QFN packages by cutting through only the plastic material.

CLAIM OF PRIORITY

This application is a divisional of and claims priority to U.S. patentapplication Ser. No. 14/301,942 (TI-74461) filed on Jun. 11, 2014, theentirety of which is incorporated herein by reference.

BACKGROUND

Accompanying the popularization of cell phones, notebook personalcomputers, and other small electronic devices has been a great demand toreduce the size and thickness of the semiconductor devices carried inthem. Together with BGA (Ball Grid Array) packages and SON (SmallOutline Non-leaded) packages, QFN (Quad Flat No lead) packages havebecome a popular configuration for small integrated circuit (“IC”)packages with leadframes.

In QFN package formation, a leadframe strip containing multipleconnected leadframes has multiple semiconductor dies mounted on it. Thisassembly is sealed with mold compound resins. Singulation is performedon this assembly with a dicing device to form individual packages. Thedicing device has a rotating blade, and, by moving the blade alongpreset cutting line, leads exposed on the bottom surface of the packageand the resin are cut simultaneously.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-7 are schematic cross-sectional elevation views illustratingvarious stages of a prior art QFN production process.

FIG. 8 is a top plan view of a die and conductive pads mounted on asupport sheet.

FIG. 9 is a cross-sectional elevation view of the assembly of FIG. 8.

FIG. 10 is a top plan view of another configuration of conductive padsof a QFN package.

FIG. 11 is a cross-sectional elevation view of the assembly of FIGS. 8and 9 after application of a first mold encapsulation layer.

FIG. 12 is a cross-sectional elevation view of the assembly of FIG. 11after application of a mold release sheet to a top surface thereof.

FIG. 13 is a cross-sectional elevation view of the assembly of FIG. 12with the support sheet and the mold release sheet being removedtherefrom.

FIG. 14 is a cross-sectional elevation view of the assembly of FIG. 13with the support sheet and mold release removed therefrom.

FIG. 15 is a cross-sectional elevation view of the assembly of FIG. 14in a flipped over position and with bond wires connecting the IC die andtwo conductive pads.

FIG. 16 is an expanded cross-sectional elevation view of the assembly ofFIG. 15, that also shows an adjacent die and wire bonded leads, afterapplication of a second mold encapsulation layer and a second moldrelease sheet to the assembly.

FIG. 17 is a cross-sectional elevation view of the assembly FIG. 16showing the mold release sheet being removed therefrom.

FIG. 18 shows the assembly of FIG. 17 being diced (singulated).

FIG. 19 is a cross-sectional elevation view showing the assembly ofFIGS. 17 and 18 after being diced into two QFN packages.

FIG. 20 is a top plan view of one of the QFN packages of FIG. 19.

FIG. 21 is a bottom plan view of one of the QFN packages of FIG. 19.

FIG. 22 is a flow chart of a method of producing a plurality of QFNpackages.

DETAILED DESCRIPTION

This specification, in general, discloses a quad flat no lead (“QFN”)package 172, FIG. 19, that includes a die 120 having an active side 122positioned substantially in a first plane AA and a backside 124positioned substantially in a second plane BB parallel to the firstplane AA. The QFN package 172 also includes a plurality of separateconductive pads 130 each having a first side 132 positionedsubstantially in the first plane AA and a second side 134 positionedsubstantially in the second plane BB. Mold compound 150 is positionedbetween the first and second planes in voids between the conductive pads130 and the dies 120. Also disclosed, in general, as illustrated in FIG.22, is a method of producing a plurality of QFN packages. The methodincludes forming a strip of plastic material having embedded therein aplurality of dies and a plurality of conductive pads that are wirebonded to the dies, e.g., FIG. 18. The method further includessingulating the strip into a plurality of QFN packages by cuttingthrough only the plastic material, e.g., FIG. 19.

FIGS. 1-7 illustrate the prior art method of forming QFN packages. FIG.1 is a cross sectional view of a leadframe strip 11 that is supported ona leadframe support tape 10. The leadframe strip 11 comprises aplurality of interlinked leadframes 12 (only two leadframes 12 are shownin FIG. 1). Each leadframe 12 has a die attach pad 14 surrounded by aplurality of leads 16. Each of the leads 16 of one leadframe 12 isintegrally connected to a lead 16 of an adjacent leadframe 12.

In a die attach process, as illustrated in FIG. 2, dies 20 are attachedto the die attach pads 14 of each leadframe 12. Each die 20 has anactive or front side 22 and a backside 24. Each die 20 is mounted on adie attach pad 14 with the backside 24 of the die facing down. Each die20 is attached to the associated die attached pad 14 with an attachmentlayer (not shown) such as solder or epoxy.

Next, as illustrated in FIG. 3, each die 20 is electrically connected tothe associated leads 16 by bond wires 26. This assembly is thenencapsulated in mold compound 30, using a support film 32, asillustrated in FIG. 4.

As illustrated in FIG. 5, next, the support tape 10 and support film 32are removed from the assembly. Then, as illustrated in FIG. 6 theassembly is diced (singulated) with a conventional singulation saw blade34 or punch or stealth laser (not shown). During singulation, integrallyattached leads 16/16 are severed by the cutting blade 34. In anotherprior art embodiment (not shown) two parallel cuts are made to separateintegrally attached leads to thereby remove a central connection portionpositioned between adjacent leadframes.

As shown by FIG. 7, singulation creates a plurality of individual QFNpackages 42, 44. Each QFN package 42, 44 has a die attach pad 14 andleads 16 of a single leadframe exposed on the bottom surface of thepackage.

During dicing metal burrs may be generated on the cutting plane of theleadframe. Such burrs are generated not only in the direction in whichthe blade moves (i.e., the cutting direction), but also in the platethickness direction (perpendicular to the top and bottom surfaces of theleadframe).

Because the clearance between leads is small in QFN packages, the burrsin the cutting direction may lead to short-circuit problems, such assolder bridges, etc., when leads are assembled on a wiring substrate.Also, because the burrs in the plate thickness direction of the leadsprotrude from the bottom surface of the package, the planarization(flatness) of the QFN package may be poor, and when it is assembled on awiring substrate, the connections to the lands of the wiring substratemay also be poor.

A new method of producing QFN packages is illustrated in FIGS. 8-19.FIGS. 8 and 9 illustrate a support tape 110 having a flat tacky topsurface 112 and a smooth (non-tacky) bottom surface 114. An integratedcircuit (“IC”) die 120 having an active front side surface 122 and abackside surface 124, which is typically metal coated) is mounted activeside 122 down, as by a pick-and-place machine, on the tacky top surface112 of the support tape 110. A plurality of metal conductive pads 130(sometimes referred to in the art as “contact pads”) having a firstsurface 132 and an opposite second surface 134 are mounted, as by apick-and-place machine, in encompassing relationship with the die 120,first surface 132 down. Each conductive pad 130 has the same height(thickness) as the die 120. Thus, the active surfaces 122 of the dies120 and the first surfaces 132 of the conductive pads 130 all liesubstantially in the same plane AA and the backside surfaces 124 of thedies 120 and the second surfaces 134 of the conductive pads 130 all liesubstantially in a second plane BB parallel to the first plane AA. Asshown by FIG. 8, each conductive pad 130 may be spaced at apredetermined distance 136 from the associated die 120, and conductivepads 130 on each side of the associated die 120 may be spaced at apredetermined distance 137 or 139 from an adjacent conductive pad 130.

The assembly shown in FIGS. 8 and 9 includes only a portion of thesupport tape 110 and the dies 120 and conductive pads 130 associatedwith that portion. The entire support tape 110 supports many such dieand conductive pad configurations. It is noted that the conductive pads130 are not portions of a leadframe and are not integrally connected toadjacent conductive pads. In one embodiment, as illustrated in FIG. 8there is no connection at all between adjacent pads 130 when they areinitially mounted on the support tape 110. In another embodiment, asillustrated in FIG. 10, adjacent pads 130 are connected by anonelectrical material 140, such as an organic material. This connectingmaterial 140 may facilitate mounting of the conductive pads 130 on thesupport tape 110 at a predetermined spacing distance from each die andat a predetermined spacing distance from adjacent conductive pads 130.

The next step in the process, as shown by FIG. 11, is to apply a firstlayer of mold compound 150. The mold compound 150 may be plastic, e.g.,epoxy. The mold compound 150 is applied to the support sheet 110 suchthat it surrounds the attached dies 120 and conductive pads 130. Themold compound 150 may be applied by using a conventional transfer mold.Mold compound 150 is applied to a depth equal to the height of die 120and the conductive pads 130, i.e., to the height of the second plane BB.The mold compound 150 fills all of the voids between the conductive pads130 and the dies 120 and all of the voids between conductive pads. As aresult the backside surface 124 of each die 120 and the second surface134 of each conductive pad are exposed, i.e., are not covered with moldcompound 150. Then, as shown by FIG. 12, a mold release film or tape 154having a top surface 156 and a bottom surface 158 is applied to theassembly on the side thereof opposite the support tape 110. This firstlayer of mold compound 150 is then allowed to cure, causing it to adhereto the die 120 and conductive pads 130 and to hold them apart at theirpredetermined spacing distances 136, 137, and 139, FIG. 8.

The next procedure, as illustrated by FIG. 13, is to remove both thesupport tape 10 and the mold release tape 154 from the assembly. At thispoint, as shown by FIG. 13, both the backside surface 124 and the activefront side surface 122 of each die 120 are exposed as are both the firstand second sides 132, 134 of the conductive pads 130. Next the entireassembly of FIG. 13 is flipped over into the orientation shown in FIG.14, as indicated by arrows 160. In this orientation shown in FIG. 14,the active front side surface 122 of each die 120 is positioned upwardlyand the first surface 132 of each conductive pad 130 is positionedupwardly.

The next procedure is to electrically connect each die 120 with itsassociated conductive pads 130. This is done, as shown by FIG. 15, bywelding a first end of each of a plurality of bond wires 162 to theactive surface 122 of each die and by welding the second end of each ofthe bond wires 162 to the first surface 132 of each associatedconductive pad 130.

After bonding the dies 120 to associated conductive pads 130 the entireassembly is again molded as shown in FIG. 16. In this second moldingoperation a second layer of mold compound 151 is applied over the firstlayer 150 producing a twice molded assembly. A second strip of moldrelease film 164 is then applied to the top of the twice molded assemblyas shown by FIG. 16. The two mold layers 150, 151 bond together due tothe heat associated with the second molding operation.

The next step, as illustrated by FIG. 17 is to remove the second stripof mold release film, after the mold compound 151 has cured. At thispoint, if the QFN packages are to be marked with indicia, marking areperformed, as with a laser, by printing or by some other means.

The next step, as illustrated by FIG. 18, is to make a series ofsingulation cuts with singulation saws 171, 173 to dice the strip 170into a plurality of IC packages 172 174, etc., as shown by FIG. 19. FIG.20 is a top plan view of one of the QFN packages 172 and FIG. 21 is abottom plan view thereof. As illustrated by FIG. 20 the top surface 182of the package 172 has a generally flat smooth face, which may have asquare or rectangular shape. The top surface 182 may have indiciaprinted thereon (not shown), as previously discussed. As shown by FIG.21 the bottom face of the QFN package 172 has the second surface 134 ofeach conductive pad 130 exposed thereon. The metal coated backsidesurface 124 of the corresponding die 120 is also exposed.

Although certain specific embodiments of a QFN package and method ofproducing a QFN package have been expressly described herein, it will beobvious to those skilled in the art, after reading this disclosure, thatthe various inventive concepts disclosed herein may be otherwiseembodied. The appended claims are intended to be construed broadly toencompass such alternative embodiments, except to the extent limited bythe prior art.

What is claimed is:
 1. A method of producing a plurality of QFN packagescomprising: forming a strip of plastic material having embedded thereina plurality of dies and a plurality of conductive pads that are wirebonded to the dies; and singulating the strip into a plurality of QFNpackages by cutting through only the plastic material.
 2. The method ofclaim 1 wherein said forming a strip of plastic material having embeddedtherein a plurality of dies and a plurality of conductive pads that arewire bonded to the dies comprises: providing a plurality of dies and aplurality of conductive pads that have substantially the same thicknessand providing a support film sheet; and attaching a first side of theplurality of dies and a first side of the plurality of conductive padsto one side of the support film sheet.
 3. The method of claim 2, saidforming a strip of plastic material further comprising filling voidsbetween the plurality of dies and the plurality of conductive pads withmold compound while leaving a side of each of the plurality of dies andeach of the plurality of conductive pads positioned remotely from thesupport sheet exposed.
 4. The method of claim 3, said forming a strip ofplastic material further comprising removing the support film sheet toprovide a strip of mold compound having a first side on which the firstside of each die and the first side of each conductive pad are exposedand a second side on which the second side of each die and the secondside of each conductive pad are exposed.
 5. The method of claim 4, saidforming a strip of plastic material further comprising wire bonding anactive side of each die to one side of each of the plurality ofconductive pads.
 6. The method of claim 5, said forming a strip ofplastic material further comprising covering the side of the strip ofmold compound having the wire bonds with a second layer of mold compoundto form a twice molded assembly.